Physics
Most quantum physics research to date has used particles such as atoms and electrons to observe quantum mechanical behaviour. Professor Mika Sillanpää of Aalto University is now working in the relatively new field of using supercool temperatures to observe quantum features in larger objects.
Micromechanical oscillators, resembling a miniaturised guitar string, are cooled down to a tem-
perature near absolute zero at -273 centigrade. The resonators exchange energy in a similar way to a guitar string and echo chamber. The energy source is provided by a microwave laser. Image by Juha Juvonen.
behaviour at the macro-scale Quantum-mechanical
When considering tiny constituents of matter, such as single atoms or molecules, the laws of physics seem to contradict common sense. Atoms or small elementary particles can properly be understood only by quantum physics, which tells that matter and energy consist of small packets, quanta. On the other hand, according to quantum physics, they both can also behave as waves. Without such detailed knowledge of the fundamental laws of nature, modern electronics, for example, could not have been constructed. Professor Mika Sillanpää of the
Department of Applied Physics and O.V. Lounasmaa laboratory at Aalto University is carrying out basic research on micromechanical resonators measured at ultralow temperatures. He was recently awarded the prestigious ERC Consolidator Grant for the years 2015-2019, worth €2m, for his project titled “Cavity quantum phonon dynamics”. Research into this field has been quite
active recently. “There was a race amongst researchers to observe the quantum ground state of the vibrations of a near-macroscopic
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object,” says Sillanpää. “This was achieved for the first time a few years ago, which further intensified research in this field and has formed the basis for our current work.” Since everything is built with atoms,
macroscopic sized objects should, in principle, follow the counterintuitive quantum laws. Quanta are never directly observed, because the quantum waves in sizable objects usually immediately cancel each other out, leaving behind the everyday world. However, if well protected from noise of the surroundings, tangible objects can retain some quantum features. “We use quite sophisticated cryogenic equipment to cool our samples close to -273°C, known as absolute zero,” Sillanpää explains. “At this temperature, the energies of single vibrational quanta are not excessively disturbed by random motion of atoms due to temperature. This allows us to observe quantum-mechanical behaviour in relatively macroscopic objects such as the micromechanical oscillators that we work with.” In Sillanpää’s work, the micromechanical resonators are housed inside a
superconducting cavity resonator. When the two quantum resonators, are put together, they begin to exchange quanta, and their resonant motion thus becomes amplified. This is very similar to what happens in a guitar, where the string and the guitars’ echo chamber resonate at the same frequency, but instead occurring in the realms of quantum physics. Instead of the musician playing the guitar string, the energy source is provided by a microwave laser.
Quantum computing Recently, Sillanpää’s group successfully connected a superconducting quantum bit, or qubit, with a micrometer-sized drumhead and transferred information from the qubit to the resonator and back again. “This work represents the first step towards creating exotic mechanical quantum states,” he states. “For example, the transfer makes it possible
to create a state in which the
resonator simultaneously vibrates and doesn’t vibrate.” A qubit is the quantum-mechanical
equivalent of the bits used in computers. A traditional bit can be in a state of 0 or 1,
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